In displaying devices for word processors, personal computers, TV's and gauges, an image or letter is observed through a transparent protective plate such as glass or plastic plate, which serves to protect the surface of the devices. Recently, most displaying devices are liquid crystal displaying devices. The liquid crystal displaying devices use two polarizing plates to display a liquid crystal image based on electronic signal. Generally, a polarizing plate comprises a polarizing film and a protective film provided thereon which protects the polarizing film. The polarizing film is obtained by stretching in a specific direction a polyvinyl alcohol (hereinafter referred to as PVA) film which has been applied with iodine and/or two color type dye.
The protective film of the polarizing plate referred to herein means a plastic film, which serves various functions and protects a polarizing film, provided on one or each side of the polarizing film.
Generally, the plastic film has a problem such as poor scratch resistance, antistatic property and chemical resistance. The above problem results in poor yield in the production of polarizing plates or liquid crystal displaying devices or in low reliability of products. For example, malfunction of the liquid crystal displaying device due to static electricity results in irreversible defect as products.
In Japanese Patent O.P.I. Publication No. 5-162261 is disclosed a protective film of a polarizing plate, the film providing good scratch resistance and having a layer, in which a UV hardenable resin layer is hardened, on a cellulose triacetate film as a plastic film. However, in this method, as the coating layer thickness is increased to enhance the surface hardness, it is likely to cause cracks and adhesion between the polarizing plate and the coating layer deteriorates under high temperature and high humidity, since saponification can not be conducted.
In Japanese Patent O.P.I. Publication No. 6-51122 is disclosed a method providing, on the protective film, an anti-static coating layer containing conductive fillers such as carbon black. There has been also used an anti-static coating containing an ion-complex type surfactant.
However, in the anti-static coating layer containing conductive fillers, the fillers appear as foreign matters, resulting in high haze and product value deterioration. In the anti-static layer containing an ion-complex type surfactant, the surface specific resistance is likely to vary depending on humidity, resulting in poor durability.
In Japanese Patent O.P.I. Publication No. 6-123806 are disclosed various ionic polymer compounds as anti-static agents, however, they provide insufficient scratch resistance.
Since the conventional polarizing film of the polarizing plate is poor in scratch resistance, the film is likely to be scratched and to catch (attract) foreign matter in the manufacturing process, which results in poor yield or a faulty product. Therefore, the polarizing film is covered with a temporary protective film to protect, and in the successive process, the temporary film is peeled from the polarizing film. However, this method has a problem in that when the temporary film is peeled from the polarizing film, the polarizing film attracts other foreign matter due to static electricity generation.
In the above described polarizing plate, strong adhesion between the polarizing film and the protective film are required in order to obtain high reliability and durability under various circumstances. In order to enhance this adhesion, a cellulose triacetate (hereinafter referred to as TAC) film used for a protective film, whose surface is saponified with an alkaline solution, is laminated with a polarizing film having a polyvinyl alcohol type adhesive to obtain a polarizing plate. However, since this method comprises immersion the TAC film in 5% or more sodium hydroxide solution for a long time for saponification, which is carried out employing a concentrated alkaline solution at high temperature, it is not favorable in view of manufacturing safety, environmental protection or cost. Further, the alkaline processing causes bleed-out on the TAC film surface of plasticizer or an increase of haze, resulting in a poor quality product.
In Japanese Patent O.P.I. Publication No. 6-94915 there is a description regarding adhesion between the polarizing film and the protective film in which the saponification process can be omitted by employing a hydrophilic polymer compound having strong adhesion property.
As is described above, a protective film of a polarizing plate is required to have excellent antistatic property, scratch resistance, adhesion, anti-glareness and chemical resistance. However, a polarizing film or polarizing plate having the above characteristics has not been obtained.